1. Field of the Invention
The invention relates to a spindle device used for a machine tool, or the like.
2. Discussion of Background
For example, in a spindle device of a machining center, an axial preload is applied to a bearing that supports a spindle in order to maintain the rotational accuracy of the spindle and the stiffness of the spindle. A preload changing system has been suggested as the system for applying a preload. The preload changing system changes a preload in response to the rotation speed of the spindle. For example, Japanese Patent Application Publication No. 60-139911 (JP-A-60-139911) describes a preload changing system. The preload changing system includes driving means for axially pressing an outer ring of a bearing based on the rotation speed of a spindle. Only when the rotation speed of the spindle exceeds a predetermined value, the preload changing system uses the driving means to press the outer ring of the bearing to thereby displace the outer ring. Thus, a difference in dimension between inner and outer spacers is reduced. Thus, a preload applied to the bearing is reduced.
In the preload changing system described in JP-A-60-139911, a preload is merely reduced in a stepwise manner with an increase in the rotation speed of the spindle, and the type of machining and the state of machining by a tool attached to the spindle are not taken into account at all.